4l80e no 4th gear

1995 GMC Suburban K2500 454......had a trans line leaking while I was driving and I didn't know it until I heard and felt a loud clunk, and suddenly lost 4th gear. I pulled over and discovered that the line was leaking and had it towed home. Next day I repaired the line and refilled the trans, all of the gears work fine, and the shifting is smooth, and it drives fine, but it no longer goes into 4th, the torque converter will lock up for about a second or two, then unlock around 45mph. I'm fearing that the loss of fluid resulted in 4th gear letting go.....any suggestions? or should I just look into a rebuild or a new trans? Thanks, Scott